This invention relates to an improved RF plasma gun and method of producing an RF plasma plume whereby a constriction of the plasma plume due to the introduction of hydrogen in such guns is reduced while also reducing heat losses from the plasma plume to a powder injection probe in the gun.
Radio frequency (RF) plasma deposition is a plasma spray process which is well known for producing high temperature gaseous plasma. The devices for generating the plasma are sometimes referred to as plasma guns. They find utility in diverse heating applications such as high temperature chemical reactions, heating of solid targets, melting of particles such as a superalloy and for providing surface coatings and spray processes. Plasma processes are also used to produce low interstitial content titanium, refractory metal, and superalloy deposits. In addition, the deposition efficiency of materials sprayed by the RF plasma process can approach 100%.
RF plasma deposition is a plasma spray process which can be used to fabricate low interstitial content titanium, refractory metal, and superalloy deposits. For example, U.S. Pat. No. 4,805,833, the disclosure of which is incorporated herein by reference, describes an RF plasma apparatus, including an RF plasma gun and the operation thereof in a frequency range of from 2 to 5 megahertz. The plasma is produced by induced RF energy which causes gases flowing in the interior of the gun to form a plasma plume or jet which flows to the adjacent substrate. Gases introduced into the plasma gun to form the plasma are herein referred to as "plasma gas." Typical plasma gas is comprised of argon, nitrogen, helium, or mixtures thereof.
Small quantities of hydrogen can be employed in the plasma gas to enhance heat transfer. Hydrogen has a low dissociation temperature, and the latent heat of dissociation of hydrogen increases the enthalpy of the resulting plasma. However, largely as a consequence of the large increase in thermal conductivity of hydrogen through dissociation at about 3000 to 4000 K., the plasma plume generated in such guns suffers from a constriction effect shown graphically in FIG. 1 (Sides A&B) which illustrates the typical temperature distribution in a plasma gas having added hydrogen and a non-dissociating (non-molecular) gas such as argon. Because of the constriction effect, coupling is weakened between the plasma and the electromagnetic field from the induction coil in the gun. This puts a limit on the mole fraction of hydrogen which can be introduced in the gun. Further, the increased thermal conductivity resulting from the dissociation of hydrogen increases heat losses to the powder injection probe.
The above described constriction of the plasma plume from the introduction of hydrogen gas to the plasma is herein referred to as "discharge constriction".